1. Field of the Invention
The invention relates to a pressure medium supply arrangement for a continuous variable transmission (CVT transmission) comprising a pump by which the control cylinder can be supplied with pressure medium (hydraulic medium) for changing the gear ratio, wherein the pump has correlated therewith a control valve arrangement via which a bypass channel between the high-pressure side and the low-pressure side of the pump can be opened or closed for controlling the conveyed flow.
2. Description of the Related Art
Such CVT transmissions have been known for several years and allow a continuous change of the gear ratio of a motor vehicle. Since the CVT transmissions have not been able to compete with conventional automatic transmissions, efforts are currently undertaken in the process of developing high torque diesel engines (common rail fuel injection system, monobloc injection pump and nozzle injection systems) to use the advantages of the CVT transmission, i.e., the possibility of a continuous change of the gear ratio as a function of the operational parameters of the motor. Such transmissions allow, for example, to maintain the engine within predetermined torque or output ranges as a function of the driving conditions (acceleration, steady driving, breaking etc.) so that an optimal output with optimal fuel consumption is ensured.
FIG. 1 shows a basic concept of a CVT transmission as, for example, disclosed in the European patent document 0 271 109 or European patent document 0 076 552.
The central part of the CVT transmission 1 illustrated schematically in FIG. 1 is a belt or piston drive 2 by which the crankshaft of an internal combustion engine 4 is connected with the reducing gear transmission 6 of a motor vehicle. The ratio of speeds of the belt drive can be adjusted continuously, for example, in the range of 3:1 up to 0.3:1 so that the motor 4 as a function of the desired driving state can always be maintained within the optimum torque/output range.
The belt drive has a belt 10 which is guided on two V pulleys 12, 14. The upper V pulley 12 of FIG. 1 is coupled to the motor 4 while the lower V pulley 14 is connected to the reducing gear transmission 6. The belt 10 is comprised conventionally of circulating steel belts which have a plurality of plate-shaped elements 16 by which the belt 10 rests on the V pulleys 12 or 14.
Each of the V pulleys 12, 14 has a stationary disc 18 or 19 and a movable disc 20 or 21 which can be moved in the axial direction relative to the correlated stationary disc 18 or 19 in order to change the support width for the belt 10 or, in other words, for the elements 16. The movement of the movable discs 20, 21 is performed by control cylinders 22, 24 wherein, for example, the control cylinder 22 correlated with the V pulley on the motor side has a greater end face surface then the cylinder 24 on the reducing gear transmission side. The control cylinders 22, 24 are connected by a control device 26 and a control valve arrangement 28 to a pump 30 and a hydraulic medium tank T.
As can be taken from the schematic of FIG. 1, the two movable discs 20, 21, respectively, the stationary discs 18, 19 are arranged in a crossed arrangement so that the belt 10 is guided always straight, i.e., in a perpendicular line to the axes of the V pulleys 12, 14.
The control device 26 comprises, for example, a two-edge or four-edge control slide (piston) by which the hydraulic medium supplied by the pump 30 can be distributed to the two control cylinders 22, 24 in order to perform the desired adjusting movements. When, for example, the same pressure is present, the support width of the V pulley 12 at the motor side is reduced by the greater force of the control cylinder 22 and, accordingly, the support width of the V pulley 14 at the reducing gear transmission side is enlarged against the force transmitted by the control cylinder 24 so that the belt 10 in the representation according to FIG. 1 moves upwardly. This is so because of the different size of the end faces of the control cylinders 22 and 24.
Via the control valve arrangement 28 the conveyed flow of the pump 30 is adjusted so that at all times only the required amount of hydraulic medium flows to the control cylinders 22, 24 or is returned from them into the tank T.
The pumps 30 are often in the form of rotary piston pumps, for example, internal gear pumps, because they have a comparatively simple construction and can produce relatively high pressures with minimal weight. A further advantage of internal gear pumps is that they can be used in comparatively broad rpm (revolutions per minute), temperature, and viscosity ranges and operate with minimal pulsation and noise.
In European patent document 0 445 529 B1 an internal gear pump is described in which two gear wheels are rotatably supported in the housing. The displacement chambers are delimited by meshing tooth flanks, an inner wall of the housing, and an axial plate or pressure plate. The latter is loaded on its backside by a hydraulic medium so that it contacts sealingly the end faces of the gear wheels. The pressure acting on the backside of the pressure plate can be adjusted by a control device in which a metering orifice embodied as a directional control valve and a three-port directional pressure regulator (pressure scale) cooperate. It is loaded in the sense of increasing the pressure at the backside by the pressure downstream of the metering orifice as well as a pressure spring and in the sense of a reduction of the pressure at the backside by the pressure upstream of the metering orifice. The pressure regulator (pressure scale) connects forth is purpose a control connector connected to a pressure chamber on the backside of the pressure plate with the pressure outlet of the pump or with the tank, so that the sealing gap between the pressure plate and the end faces of the gear wheels is reduced or enlarged. For an enlarged sealing gap the hydraulic medium can flow directly from the high pressure side to the low pressure side so that the volumetric efficiency of the gear wheel machine can be reduced and, for example, the conveying volume or capacity of the gear pump can be adjusted as a function of the gap width. This means that by influencing the hydraulic medium pressure acting on the pressure plate an exact conveyed flow regulation can be performed.
In U.S. Pat. No. 4,014,630 a pump arrangement is disclosed in which a sealing plate is forced by a spring into the contact position on the conveying elements. The spring chamber is connected by a throttle bore to the displacement chamber of the pump arrangement so that the pressure plate is also hydraulically prestressed into its sealing position. The pressure acting on the pressure plate can again be adjusted by a control device via which the hydraulic medium can be returned to a tank.
A disadvantage of this construction is that a considerable expenditure must be provided in order to be able to adjust the conveyed flow of the pump quickly to changes of the operating parameters of the motor.
It is an object of the present invention to provide a hydraulic medium supply for a CVT transmission which allows a fast adjustment of the conveyed flow to the changes of the operational and functional parameters of the motor of a vehicle.
In accordance with the present invention, this is achieved in that the control valve arrangement is electrically controlled.
According to the present invention, the control valve arrangement correlated with the pump is electrically controlled so that the conveyed flow change can be adjusted already beforehand, i.e., for example, upon initiation of the acceleration or braking process. This means that via suitable sensors changes of the corresponding control members (gas pedal, fuel pump, diesel pump, brake pedal etc.) are detected and, as a function of these changes, a signal is sent by the motor control to the control valve arrangement so that the fastest possible adjustment of the conveyed flow of the pump can be carried out.
The hydraulic medium supply arrangement according to the invention is preferably embodied such that the adjusting pump is a rotary piston pump (preferably an internal gear pump) in which the displacement chambers at their end faces are sealed by a pressure plate. This pressure plate is hydraulically pretensioned into its sealing position so that the sealing gap can be adjusted by changing the pressure acting on the backside of the pressure plate. When the sealing plate is in its lifted position, the pressure medium can flow directly from the high-pressure area of the pump into the low-pressure area of the pump so that the conveyed flow can be lowered. For changing the pressure acting on the pressure plate, three concepts are favored according to the present invention.
In the first variant, the adjustment of the conveyed flow is realized by a two edge flow control in which a pressure scale (pressure regulator) is loaded by the pressure downstream and upstream of a metering orifice and is electrically adjustable in order to connect a control connector, communicating with the backside of the pressure plate, to the return connector for the purpose of a control pressure reduction or to the pump for the purpose of an increase of the control pressure.
This pressure scale (pressure regulator) connects in one end position the pressure connector and the return connector to the tank so that the pressure plate is pressure-relieved and a fast adaptation to changes of the conveyed flow is possible.
The control arrangement can be designed especially compact when the metering orifice is a metering orifice bore provided in the piston of the pressure scale (pressure regulator). The piston furthermore has two control edges by which the connection of the control connector to the tank connector and to the pressure connector can be opened and closed. Such a configuration is disclosed in German patent application 199 17 593 published after the priority date of the instant application; its disclosure is incorporated into the instant application by reference.
Alternatively, the conveyed flow of the pump can also be realized by a single edge flow control or a single edge pressure control.
The latter is carried out, for example, by an electrically actuated pressure limitation valve by which the control pressure acting on the backside of the pressure plate is limited to a maximum pressure. This control pressure is taken from the high-pressure side of the pump by an inlet aperture and is supplied to the backside of the pressure plate.
In the single edge flow control a two port pressure scale (pressure regulator) is provided, which is loaded, on the one hand, by the pressure upstream of the measuring orifice arranged in the pressure line and, on the other hand, by the force of a solenoid and the pressure downstream of the measuring orifice wherein the two latter force components (solenoid and downstream pressure) act in the closing direction. Upon increase of the pressure differential across the measuring orifice, the pressure scale (pressure regulator) is moved into its through (open) position in which the control pressure is relieved in direction to the tank T so that the pressure plate is lifted off and moves into its open position.